The present invention relates to a moisture meter. More particularly, it relates to an electrostatic type moisture meter having a resonant or resonance circuit, and a measuring electrode connected in parallel therewith, said resonance circuit including a variable capacitance element. The moisture meter takes a precise measurement of an increase in capacitance across the electrode, and hence measures the water content in accordance with the increase in capacitance of the variable capacitance element.
Since the water content of a sample is proportional to the capacitance thereof, a measurement of the latter provides the water content therein, as is known. Thus, among the conventional moisture meters, a typical meter has a resonance circuit and a measuring electrode connected in parallel therewith and having a sample such as sand, between the electrodes, the capacitance of the sample is found through the measurement of the change in capacitance of the resonance circuit, thus permitting the measurement of the water content in the sample based on the linearity between the capacitance and the water content.
However, the moisture meter of this type has disadvantages in some respects. For example, since a bridge circuit is most widely used to find the change in capacitance across the electrode in the conventional moisture meter, an unbalanced bridge output voltage is phase-detected to remove any influence of a resistance involved therein, thus finding an output in proportion to the change in capacitance. When the resistance varies in a small range, it has only a negligible influence upon the precise measurement of the capacitance, but when the resistance varies in a wide range, the influence due to the resistance is not negligible. Then, a measurement without the influence of the resistance may be conducted by connecting the electrode in parallel with the resonance circuit having a variable capacitance element, and replacing the change in capacitance across the electrodes with the change in capacitance in the variable capacitance element. A variable capacitor is most readily used as a variable capacitance element in the resonance circuit. The use of the variable capacitor has in fact an advantage of accuracy in measurement of the change in the capacitance. However, as is apparent, the variable capacitor has to be operated by hand only with inconvenience or by a servomotor which will inevitably make the apparatus larger and more expensive.
Furthermore, the conventional moisture meter has a coaxial cable for connecting the electrode with the main portion of the moisture meter. The capacitance of the cable greatly varies with the change in ambient temperature, thus resulting in a large temperature drift of the resonance circuit inclusive of the cable. This drift is further accompanied by drifts in other elements such as inductance and capacitance due to the change in ambient temperature. Thus, as is apparent, a great difficulty is encountered with in the measurement of only the change in capacitance corresponding to the water content in the sample between the electrodes. In addition, so far as the electrode is connected with the main portion of the moisture meter with the coaxial cable, a plurality of meters are needed to measure the water content at a plurality of points.